This invention relates to producing "attic oil" from between the top of a subterranean reservoir and the uppermost opening into a well which encounters the reservoir. The invention utilizes a chemically-aided expanding gas cap drive.
Numerous reservoirs are tilted or curved so that oil and/or gas becomes trapped against a fluid-impermeable upper closure. Although such traps may contain significant amounts of attic oil, they are apt to be too small to be feasibly tapped by a well. In general, the oil below such an attic location can be produced by a conventional process such as a natural or artificial water or gas drive, a solution gas drive, or the like. In the absence of a water drive, when the pressure of any gas which is located above the uppermost opening into a production well becomes exhausted, the flow of attic oil simply stops. In the presence of a water drive, when the water reaches the producing location, the water-to-oil ratio of produced fluid becomes uneconomically high.
The following patents relate to prior attempts to produce attic oil and/or obviate or postpone the "water coning" which may increase the water-to-oil ratio of the produced fluid even before an upward advance of a static water level has reached the production location.
U.S. Pat. No. 2,258,614 suggests injecting oil or gas in order to depress a water cone with a liquid less dense than water, then producing fluid from the reservoir at a relatively slow rate.
U.S. Pat. No. 2,784,787 suggests treating a well in which the oil overlays water that might cone by injecting a slug of sealing fluid at the oil/water interface while simultaneously injecting non-sealing fluid into the oil layer to prevent any upflow of the sealing fluid.
U.S. Pat. No. 2,788,855 suggests injecting gas into the water layer to displace the water radially away from the well and then producing fluid from the well at a higher level.
U.S. Pat. No. 4,205,723 suggests preventing water coning while producing attic oil by injecting fluid which is gaseous at the reservoir temperature, injecting a water-excluding agent into the reservoir during or after allowing the gas to migrate to the top of the reservoir, and then resuming the production of fluid.
The commonly employed process for producing attic oil involves injecting enough gas to initiate or increase the energy of a gas cap drive and, after waiting for the gas to migrate to the top of the reservoir, resuming production. This may require an undesirably long waiting period and the duration of such a waiting period increases with increases in the viscosity of the reservoir oil.
In the course of research on other well treating problems, it was found that self-reactive aqueous solutions for generating nitrogen gas could be compounded and flowed into wells so that they generate the gas and heat at selected times and rates which are useful for various well treating operations.
U.S. Pat. No. 4,178,993 by E. A. Richardson and R. F. Scheuerman describes a well treating process for initiating fluid production from a liquid-containing well by injecting an aqueous solution containing nitrogen-gas-generating reactants having a concentration and rate of reaction correlated with the pressure and volume properties of the reservoir and the well. The reactants are arranged to be capable of reacting at a moderate rate while generating enough gas to displace enough liquid from the borehole to reduce the hydrostatic pressure to less than the fluid pressure within the reservoir.
U.S. Pat. No. 4,219,083 by E. A. Richardson and R. F. Scheuerman describes a process for backflushing well casing perforations by injecting an aqueous solution containing nitrogen-gas-generating reactants, an alkaline buffer for providing a reaction-retarding pH and an acid-yielding reactant for subsequently overriding the buffer and lowering the pH in order to trigger a fast-rising pulse of heat and pressure which causes a perforation-cleaning backsurge of fluid through the perforations.
U.S. Pat. No. 4,232,741 by E. A. Richardson, R. F. Scheuerman, D. C. Berkshire, J. Reisberg and J. H. Lybarger describes a process for temporarily plugging thief zones within a reservoir by injecting an aqueous solution containing nitrogen-gas-generating reactants, a foaming surfactant, an alkaline buffer for delaying and moderating the reaction and an acid-yielding reactant for subsequently reducing the pH, arranged so that they initially delay the reaction and subsequently initiate a moderate rate of gas production in order to form a foam which is, temporarily, relatively immobile within the reservoir formation.
Patent application Ser. No. 127,355 filed Mar. 5, 1980, now abandoned, by D. R. Davies and E. A. Richardson describes a process for conducting a production test by circulating a solution of nitrogen-gas-generating reactants through conduits within a well, buffering the solution at a pH providing a promptly-initiated reaction having a relatively mild rate of reaction, with the solution being inflowed at a rate such that the gas which it generates serves as a lift gas for gas-lifting fluid from the reservoir through another conduit within the well.
Patent application Ser. No. 215,895 filed Dec. 12, 1980, now U.S. Pat. No. 4,330,037, by E. A. Richardson and W. B. Fair, Jr. describes a process for treating an oil-containing reservoir, to chemically heat the reservoir and increase its effective mobility to oil, by injecting an aqueous solution of nitrogen gas-generating reactants with the solution having a volume, a rate of reaction and a heat-generating capability such that during its injection, the heat is generated below a selected depth and a selected volume of the reservoir is heated to a selected temperature.
Patent application Ser. No. 307,035 filed Sept. 30, 1981 by E. A. Richardson and W. B. Fair, Jr. describes a process for unplugging well casing perforations submerged under a relatively dense brine by injecting a relatively fast-reacting nitrogen-gas-generating solution arranged to have a density exceeding that of the brine, in the presence or absence of an oil solvent, so that the reactive solution sinks below the brine and the plugged perforations are heated and scrubbed by the gas-generating reaction.
The disclosures of the U.S. Pat. Nos. 4,178,933; 4,219,083 and 4,232,741 patents and patent applications Ser. Nos. 127,355, 215,895, and 307,305 are incorporated herein by cross-reference.